1. Technical Field of the Invention
The present invention is related to the de-embedding of optical component characteristics from optical device measurements and, in particular, to a method and system for de-embedding the optical component characteristics using Rayleigh backscatter.
2. Background of the Invention
Ideally, an optical signal entering one side of an optical component such as a connector or splice emerges from the other side at the same signal level. In actuality, however, some loss in signal strength will be incurred. Therefore, when analyzing an optical device, it is important to first de-embed the signal loss or other characteristics of any optical components that may be present.
Rayleigh backscatter (RBS) measurements can be used for de-embedding optical component characteristics such as signal loss. When measuring RBS, the level of RBS can be expressed in terms of the reflectivity RBS of the optical fiber as follows.RRBS=S·α·Δz  (1)where S is the RBS capture ratio, α is the attenuation coefficient due to the RBS, and Δz is the spatial resolution of the RBS measurements. The capture ratio S and the attenuation coefficient α are characteristics of the optical fiber.
The spatial resolution Δz, on the other hand, is partly a function of the frequency span of the optical signal. In heterodyne measurement techniques, the best spatial resolution theoretically possible is given by:                               Δ          ⁢                                           ⁢          z                =                  c                      2            ⁢                          n              g                        ⁢            Δ            ⁢                                                   ⁢            υ                                              (        2        )            where c is the speed of light in a vacuum, ng is the group index of the fiber, and Δυ is the total frequency span of the optical signal. Thus, by expanding the frequency span Δυ, the spatial resolution of the RBS measurements may be improved
Unfortunately, using interferometic measurement techniques, the detected RBS has a noise like behavior that exists at all input power levels This noise like behavior makes it difficult to determine the difference in the level of RBS from one side of the optical component to the other. To compensate for this noise like behavior and to determine the RBS level accurately, the RBS signal must be averaged over the length of a fiber. Various time-domain averaging techniques have been used to compensate for the noise like behavior and also to improve the measurement dynamic range. However, these techniques cannot be used to directly measure optical component characteristics such as connector loss.
Another challenge in making RBS measurements using interferometic measurement techniques is to ensure that the magnitude response of the photoreceiver is flat over the desired frequency range. Non-linearity in the photoreceiver response results in variations in the photoreceiver output that prevent an accurate measurement of the RBS levels.
It is also important to ensure that the input optical signal is swept at a constant rate. A constant sweep rate means that the frequency of the input optical signal, typically from a tunable laser source (TLS), is changed at a constant rate. A non-constant sweep rate can broaden the beat frequency of the reflections that form RBS. Such broadening can make it very difficult to detect individual, closely-spaced reflections
Because of the above difficulties in making RBS measurements, optical components characteristics measured based on RBS are sometimes inaccurate. As a result, the optical component characteristics can be incorrectly de-embedded from the optical device measurements. Accordingly, it would be desirable to provide a way to improve the accuracy of RBS based optical component characteristic measurements More specifically, it would be desirable to be able to compensate for the noise like behavior of the RBS signals, and to correct for any frequency response non-linearity in the photoreceiver, as well as any non-constant sweep rate in the TLS.